Submerged gas evaporators, submerged gas reactors and combination submerged gas evaporator/reactor systems in which gas is dispersed within the liquid phase, referred to generally herein as submerged gas processors, are well known types of devices used in many industries to perform evaporation and chemical reaction processes with respect to various constituents. U.S. Pat. No. 5,342,482 discloses a common type of submerged combustion gas evaporator, in which combustion gas is generated and delivered though an inlet pipe to a dispersal unit submerged within the liquid to be evaporated. The dispersal unit includes a number of spaced-apart gas delivery pipes extending radially outward from the inlet pipe, each of the gas delivery pipes having small holes spaced apart at various locations on the surface of the gas delivery pipe to disperse the combustion gas as small bubbles as uniformly as practical across the cross-sectional area of the liquid held within the processing vessel. According to current understanding within the prior art, this design provides desirable intimate contact between the liquid and the combustion gas over a large interfacial surface area while also promoting thorough agitation of the liquid within the processing vessel.
Because submerged gas processors do not employ heat exchangers with solid heated surfaces, these devices provide a significant advantage when compared to conventional evaporators or chemical reactors when contact between a liquid stream and a gas stream is desirable. In fact, submerged gas processors are especially advantageous when the desired result is to highly concentrate a liquid stream by means of evaporation.
However, many feed streams, prior to reaching a desired concentration and/or while undergoing chemical reactions, produce solids in the form of precipitates that are difficult to handle. These precipitates may include substances that form deposits on the solid surfaces of heat exchangers used in conventional evaporators and reactors, and substances that tend to form large crystals or agglomerates that can block passages within processing equipment, such as the gas exit holes in the system described in U.S. Pat. No. 5,342,482. Generally speaking, feed streams that cause deposits to form on surfaces and create blockages within process equipment are called fouling fluids.
Additionally, common problems within conventional evaporation and chemical reaction systems used for processing fouling fluids include deterioration of the rate of heat transfer over time due to the buildup of deposits on solid heat exchange surfaces and equipment malfunctions related to blockages in critical locations such as gas outlet pipes. These common problems adversely affect the efficiency and costs of conventional processes in that the potential for buildup of deposits and blockages necessitate frequent cleaning cycles to avoid sudden failures within the evaporation or reaction equipment.
Additionally, most evaporation and chemical reactor systems that rely on intimate contact between gases and liquids are prone to problems related to carryover of entrained liquid droplets that form as the vapor phase disengages from the liquid phase. For this reason, most evaporator and reactor systems that require intimate contact of gas with liquid include one or more devices to minimize entrainment of liquid droplets and/or to capture entrained liquid droplets while allowing for separation of the entrained liquid droplets from the exhaust gas flowing out of the evaporation zone. The need to mitigate carryover of entrained liquid droplets may be related to one or more factors including conformance with environmental regulations, conformance with health and safety regulations and controlling losses of material that might have significant value.
Unlike conventional evaporators and reactors where heat is transferred to the material being processed through heat exchangers with solid surfaces, heat and mass transfer within submerged gas processors take place at the interface of a discontinuous gas phase dispersed within a continuous liquid phase. Compared to the fixed solid heat transfer surfaces employed in conventional evaporators and reactors, fouling fluids cannot coat the heat transfer surface within submerged gas processors as new surface area is constantly being formed by a steady flow of gas which is dispersed within the liquid phase and remains in contact with the liquid for a finite period of time before disengaging. This finite period of time is called the residence time of the gas within the evaporation, or evaporation/reaction zone.
Submerged gas processors also tend to mitigate the formation of large crystals because dispersing the gas beneath the liquid surface promotes vigorous agitation within the evaporation or the evaporation/reaction zone, which is a less desirable environment for crystal growth than a more quiescent zone. Further, active mixing within an evaporation or reaction vessel tends to maintain precipitated solids in suspension and thereby mitigates blockages that are related to settling and/or agglomeration of suspended solids.
However, mitigation of crystal growth and settlement is dependent on the degree of mixing achieved within a particular submerged gas processor, and not all submerged gas processor designs provide adequate mixing to prevent large crystal growth and related blockages. Therefore, while the dynamic renewable heat transfer surface area feature of submerged gas processors eliminates the potential for fouling liquids to coat heat exchange surfaces, conventional submerged gas processors are still subject to potential blockages and carryover of entrained liquid within the exhaust gas flowing away from the evaporation zone.
Direct contact between hot gas and liquid undergoing processing within a submerged gas processor provides excellent heat transfer efficiency. If the residence time of the gas within the liquid is adequate for the gas and liquid temperatures to equalize, a submerged gas processor operates at a high level of overall energy efficiency. For example, when hot gas is dispersed in a liquid that is at a lower temperature than the gas and the residence time is adequate to allow the gas and liquid temperatures to attain the adiabatic operating temperature for the system, all of the available driving force of temperature differential will be used to transfer thermal energy from the gas to the liquid. The minimum residence time to attain equilibrium of gas and liquid temperatures within the evaporation, reaction or combined reaction/evaporation zone of a submerged gas processor is a function of factors that include, but are not limited to, the temperature differential between the hot gas and liquid, the properties of the gas and liquid phase components, the properties of the resultant gas-liquid mixture, the net heat absorbed or released through any chemical reactions and the extent of interfacial surface area generated as the hot gas is dispersed into the liquid.
Given a fixed set of values for temperature differential, properties of the gas and the liquid components, properties of the gas-liquid mixture, heats of reaction and the extent of the interfacial surface area, the residence time of the gas is a function of factors that include the difference in specific gravity between the gas and liquid or buoyancy factor, and other forces that affect the vertical rate of rise of the gas through the liquid phase including the viscosity and surface tension of the liquid. Additionally, the flow pattern of the liquid including any mixing action imparted to the liquid such as that created by the means chosen to disperse the gas within the liquid affect the rate of gas disengagement from the liquid.
Submerged gas processors may be built in various configurations. One common type of submerged gas processor is the submerged combustion gas evaporator that generally employs a pressurized burner mounted to a gas inlet tube that serves as both a combustion chamber and as a conduit to direct the combustion gas to a dispersion system located beneath the surface of liquid held within an evaporation vessel. The pressurized burner may be fired by any combination of conventional liquid or gaseous fuels such as natural gas, oil or propane, any combination of non-conventional gaseous or liquid fuels such as biogas or residual oil, or any combination of conventional and non-conventional fuels.
Other types of submerged gas processors include hot gas evaporators where hot gas is either injected under pressure or drawn by an induced pressure drop through a dispersion system located beneath the surface of liquid held within an evaporation vessel. While hot gas evaporators may utilize combustion gas such as hot gas from the exhaust stacks of combustion processes, gases other than combustion gases or mixtures of combustion gases and other gases may be employed as desired to suit the needs of a particular evaporation process. Thus, waste heat in the form of hot gas produced in reciprocating engines, turbines, boilers or flare stacks may be used within hot gas evaporators. In other forms, hot gas evaporators may be configured to utilize specific gases or mixtures of gases that are desirable for a particular process such as air, carbon dioxide or nitrogen that are heated within heat exchangers prior to being injected into or drawn through the liquid contained within an evaporation vessel.
Regardless of the type of submerged gas processor or the source of the gas used within a processor, in order for the process to continuously perform effectively, reliably and efficiently, the design of the submerged gas processor must include provisions for efficient heat and mass transfer between gas and liquid phases, control of entrained liquid droplets within the exhaust gas, mitigating the formation of large crystals or agglomerates of particles and maintaining the mixture of solids and liquids within the submerged gas processing vessel in a homogeneous state to prevent settling of suspended particles carried within the liquid feed and/or precipitated solids.